1. Field of the Invention
The present invention relates to a photodetecting device having a photodetecting section including a photoelectric surface for converting incident light into photoelectrons and a semiconductor detection element for detecting the photoelectrons emitted from the photoelectric surface, and an image sensing apparatus using the photodetecting device.
2. Related Background Art
Conventionally, a vacuum tube type photodetecting device is known which has a photodetecting section having a photoelectric surface for emitting photoelectrons upon incidence of light and an electron incidence type semiconductor detection element for converting the photoelectrons emitted from the photoelectric surface into a signal voltage. As an example of a photodetecting device of this type, an electron tube is disclosed in Japanese Patent Laid-Open No. 6-243795. The electron tube disclosed in this prior art has a photoelectric surface and a back irradiation type CCD opposing the photoelectric surface. This electron tube can detect weak light or UV light, unlike a camera which has no photoelectric surface and makes light directly incident on a semiconductor detection element such as a CCD.
In recent years, however, demand has arisen for further improvement of the photodetecting efficiency of photodetecting devices represented by the electron tube disclosed in Japanese Patent Laid-Open No. 6-243795, which has a photodetecting section including a photoelectric surface and an electron incidence type semiconductor detection element.
The present invention has been made in consideration of the above situation, and has as its object to provide a photodetecting device with an improved photodetecting efficiency and an image sensing apparatus using the photodetecting device.
To achieve this object, the present inventors have completed the present invention with an emphasis on a technique of reducing a dark current flowing to the photoelectric surface and the semiconductor detection element such as a CCD.
A photodetecting device according to the present invention is characterized by comprising a photodetecting section having a photoelectric surface for emitting photoelectrons upon incidence of light, a semiconductor detection element having an electron incident surface on which the photoelectrons can be incident, and a vacuum vessel in which the photoelectric surface is arranged on one inner surface, and the semiconductor detection element is arranged on the other inner surface opposing the one surface, and cooling means for cooling a structure on the semiconductor detection element side of the vacuum vessel.
In the photodetecting device of the present invention, when measurement light becomes incident on the photoelectric surface, photoelectrons are emitted. When the photoelectrons are incident on the electron incident surface of the semiconductor detection element, the incident light is detected. In the present invention, the structure on the semiconductor detection element side of the vacuum vessel is cooled, and therefore, a dark current in the semiconductor detection element is suppressed. Not only the semiconductor detection element but also the photoelectric surface is cooled through the vacuum vessel in which the semiconductor detection element and photoelectric surface are arranged. Hence, a dark current in the photoelectric surface is also suppressed, and the photodetecting efficiency is improved.
In the photodetecting device of the present invention, preferably, the cooling means has an absorption portion for absorbing heat and a heat generation portion for generating heat, and the heat absorption portion is arranged on the semiconductor detection element side of the vacuum vessel. As such cooling means, a Peltier element is preferably used.
When this arrangement is employed, for example, the absorption portion of the cooling means such as a Peltier element absorbs heat of the semiconductor detection element. The semiconductor detection element is cooled, and the dark current is suppressed. Not only heat of the semiconductor detection element but also heat of the photoelectric surface is absorbed by the absorption portion of the cooling means through the vacuum vessel. The photoelectric surface is cooled, and the dark current is suppressed.
Preferably, the photodetecting device further comprises a housing which comprises a transparent portion capable of passing the light to be incident on the photoelectric surface, and a voltage introduction terminal capable of supplying, to the photodetecting section, a voltage to be applied between the photoelectric surface and the electron incident surface, and accommodates at least part of the vacuum vessel, and the heat generation portion of the cooling means is fixed at a predetermined position on an inner surface of the housing.
When this arrangement is employed, for example, incident light transmitted through the transparent portion made of glass reaches the photoelectric surface, and photoelectrons are generated by the photoelectric surface. When a voltage is applied between the photoelectric surface and the electron incident surface from a voltage supply source outside the housing through the voltage introduction terminal, the photoelectrons generated by the photoelectric surface are attracted by the electron incident surface. With this operation, the incident light is detected. In addition, since the heat generation portion of the cooling means is fixed at a predetermined position on an inner surface of the housing, heat generated by the heat generation portion of the cooling means is transmitted to the transparent portion through the housing. For this reason, the transparent portion is not excessively cooled, and condensation on the transparent portion is prevented. Hence, the efficiency of measurement light reaching the photoelectric surface, and more specifically, the photodetecting efficiency of the photodetecting device is improved.
When the arrangement with the housing having the transparent portion and voltage introduction terminal is employed, the housing preferably further comprises a vacuum port for setting a vacuum state in the housing.
In this case, the vacuum state is set in the housing by exhausting the gas from the housing through the vacuum port using a vacuum pump or the like. When the vacuum state is set in the housing, the cooling efficiency of the photoelectric surface and semiconductor detection element is improved, and discharge between the voltage introduction terminal and the housing can be prevented.
When the arrangement with the housing having the transparent portion and voltage introduction terminal is employed, the housing preferably further comprises a gas introduction port for introducing and sealing a dry inert gas having a pressure lower than atmospheric pressure in the housing.
In this case, since the gas sealed in the housing is a dry gas, condensation in the housing is prevented. Hence, the efficiency of incident light reaching the photoelectric surface is prevented from lowering due to condensation. Additionally, since the gas is an inert gas with a pressure lower than atmospheric pressure, discharge between the voltage introduction terminal and the housing can be prevented.
The housing further preferably further comprises radiation means for radiating heat, the radiation means being arranged at the predetermined position where the heat generation portion of the cooling means is fixed.
When this arrangement is employed, heat generated by the heat generation portion of the cooling means is externally radiated from the housing by the radiation means. Hence, the cooling efficiency of the vacuum vessel is improved, and the dark current flowing to the photoelectric surface and semiconductor detection element is further reduced.
The vacuum vessel may comprise an incident surface plate which passes the light and has the photoelectric surface arranged on one surface, a detection element fixing plate opposing the incident surface plate and having the semiconductor detection element arranged thereon, and a side tube which forms a vacuum space together with the incident surface plate and the detection element fixing plate.
When this arrangement is employed, the semiconductor detection element is cooled by the vacuum vessel through the detection element fixing plate. Since the incident surface plate on which the photoelectric surface is arranged is connected to the detection element fixing plate through the side tube, not only the semiconductor detection element but also the photoelectric surface is cooled by thermal conduction.
Especially, when the side tube and detection element fixing plate are formed from a ceramic material having a high thermal conductivity, the thermal conductivity is improved, and the photoelectric surface can be easily cooled as the semiconductor detection element is cooled.